| Expansins are wall proteins that are capable of promoting plant cell wall extension. They are encoded by a gene superfamily, which consists of four distinct families, EXPA, EXPB, EXLA, and EXLB. EXPA and EXPB were discovered as they reconstitute wall extension of the heat-inactivated cucumber hypocotyls whereas EXLA and EXLB were identified based on the sequence similarity to the first two families. The mechanism of expansin function is proposed as weakening the hydrogen bonds between cellulose microfibrils to facilitate wall loosening. Large numbers of expansins have been identified in plants from mosses to angiosperms, and in non-plants as well. However, in vivo functional study of expansins is very limited.;In this dissertation, a group of pollen-expressed Arabidopsis expansins were selected for functional study in both vegetative and reproductive growth. EXPA13 expression was observed in microsporogenesis and in the differentiating vessel elements. Reverse genetic studies were performed in three expa13 mutants. Atexpa13-1, a qrt1 background mutant with increased gene expression, produces half-collapsed tetrads in the heterozygous mutant, in which wild-type pollen tends to break. It is proposed that imbalanced microspore expansion in heterozygous tetrad leads to the phenotype. No significant change in pollen size was observed in two other knockout mutants, atexpa13-2 and atexpa13-3. The attempts to obtain expa13 qrt1 double mutant by crossing atexpa13-2 and qrt1-1 plants have failed.;Deficient vegetative growth has been observed in EXPA13 knockout mutant with differential severities on different ecotypes. Transcript analysis and mutant observation suggest that EXPA13 controls cell elongation as well as wall thickening during xylem element development. Combining the phenotypic changes in microsporogenesis and stem development, I conclude that EXPA13 facilitates microspore expansion and xylem element elongation.;EXPA4, EXPA24, and EXPB5 are expressed in Arabidopsis mature pollen grains and elongating pollen tubes. This indicates that both alpha- and beta-expansins are functioning in the reproductive development of Arabidopsis. Their expression profiles were constructed, and knockout mutants have been identified for EXPA4 and EXPB5, named atexpa4-1 and atexpb5-1 respectively. No phenotypic changes were noticed in either line. No EXPA24 mutant has been isolated from public stocks. The biological function of Arabidopsis pollen-expressed expansins is hypothesized as facilitating wall assembly of elongating pollen tube and wall separation of the female tissues as the pollen tubes pass through. |
